This invention relates generally to radio frequency limiter circuits and more particularly to radio frequency limiter circuits adapted to operate with relatively high power.
As is known in the art, radio frequency energy limiter circuits are used in a wide variey of applications. One type of limiter circuit includes a p-i-n diode connected in shunt across a transmission line. In many case an inductor is connected in shunt with the diode. As the power fed to the transmission line increases, the effective resistance (i.e. the shunt resistance, R.sub.s) of the diode decreases from a high resistance condition (whereby, in the ideal case, the power fed to the transmission line passes unattenuated to the output of the transmission line) to a low resistance condition, (whereby, in the ideal case, the power fed to the transmission line is reflected back to the input of the transmission line to limit the amount of power passing to the output of the limiter). The purpose of the inductor is to provide a discharge path during the high resistance condition for charge stored in the "i" or intrinsic region of the p-i-n diode during a preceeding time when the diode was in the low resistance condition.
In many application such limiters are required to operate with relatively high power levels. Here the breakdown voltages of the p-i-n diodes must correspondingly increase. This increase in breakdown voltage is typically achieved by increasing the volume of the intrinsic region of the p-i-n diode. However, as is known, as the intrinsic region volume is increased, the low impedance characteristic required by the diode degrades. That is, as the intrinsic region volume increases, the minimum shunt resistance achievable for the diode during the higher power level operating condition may exceed that required for the particular limiter application.
One technique used to increase the power handling capability of the diode is to use two p-i-n diodes both connected in shunt across the transmission line, but connected at points along the transmission line separated from one another by a quarter wavelength. Here the diode connected closest to the input of the limiter has a higher breakdown voltage characteristic than the diode connected closest to the output of the limiter. At low power levels both diodes in effect provided high shunt resistance conditions so that substantially all of the input power passes to the output of the limiter. As the input power increases however, while the diode connected closest to the input of the limiter maintains a somewhat higher resistance condition because of its larger intrinsic region volume, the diode connected closest to the output of the limiter is able to have its resistance reduced considerably because of its lower intrinsic region volume thereby commencing the limiting process. Further, the lowered resistance of the latter diode produces a low voltage which is reflected back to the former diode as a high voltage because of the quarter wavelength transmission line separating the two diodes. Thus, the former diode is driven to a lower resistance condition because of two effects: first, the diode is subjected to an increase in input power to the limiter; and second, the higher voltage reflected to it by the latter diode further drives it to a conducting, lower resistance condition. Thus, for high power application, the former diode, having an increased intrinsic region, is able to handle higher power levels and the concomitant ineffective low resistance charateristics of such former diode is, in effect, compensated by the use of the latter, low intrinsic region diode which provides the requisite low resistance condition.
In some high power applications it is necessary to further increase the power handling capability of the above-mentioned former diode, (i.e. the diode connected closest to the input of the limiter). Here a second high breakdown voltage diode is connected in shunt with the former diode at the same point in the transmission line. While the use of shunt connected diodes at the same point of the transmission line does reduce the power handling required for any one of the diodes, the insertion loss of the limiter is correspondingly increased. This increase in insertion loss is attributable to the fact that the intrinsic region of each diode in effect provides a capacitance in parallel with the transmission line. Thus, as additional diodes are connected in shunt, the capacitance across the transmission line increases. Since it is desirable that the input impedance to the limiter is matched to the impedance of the source of radio frequency energy or power fed to the limiter, matching circuitry must be provided; however, as the capacitance increases from using additional shunted diodes, the complexity of the matching circuit increases, and correspondingly, the operating bandwidth and maximum operating frequency of the limiter decreases thereby restricting the application of the limiter.